With the continued identification of novel genes associated with Parkinsonism an imperative question in the field is to know whether these genes interact to lead to the development of the cardinal features of Parkinson's disease. This lack of information is an important problem because it limits the understanding of potential common mechanisms underlying Parkinson's disease thus also limiting the development of novel disease-modifying treatments. The long-term goal is to improve treatment outcomes in Parkinson's disease by identifying novel therapeutic targets that can prevent or significantly slow the progression of the disease. The objective in this particular application is to determine how the genetic loci ATP13A2 (PARK9) and alpha-synuclein (PARK1) interact, specifically how does the loss of function of ATP13A2 affect alpha-synuclein toxicity in mice. ATP13A2 null and alpha-synuclein overexpressing mice each show aspects of behavioral impairment and anatomical pathology reminiscent of that seen in Parkinson's disease highlighting their usefulness. The central hypothesis is that loss of function of ATP13A2 enhances alpha-synuclein toxicity leading to accelerated dopaminergic cell loss and functional impairments in vivo. The hypothesis has been formulated on the basis of preliminary and published studies produced by the applicants' laboratories. The rationale for the proposed research is that once the pathologic mechanisms associated with cellular and behavioral dysfunction are identified, the development of potential therapies can be initiated. Guided by preliminary data this hypothesis will be tested by pursuing two specific aims: 1) Determine the functional effects of the interaction between loss of ATP13A2 and enhanced 1-synuclein protein burden in vivo. 2) To test the hypothesis that loss of function of ATP13A2 combined with overexpression of alpha-synuclein in mice leads to lysosomal dysfunction, increased abnormal accumulation of alpha-synuclein, and an acceleration of nigrostriatal dopamine neuron death in vivo. In the first aim, ATP13A2 null mice generated by the applicants' laboratory and mice that overexpress alpha-synuclein will be crossbred to generate double mutant ATP13A2 null mice that overexpress alpha-synuclein. These mice will be tested for behavioral impairments as they age on tests designed and established in the applicants' laboratories. In aim two, a combination of immunohistochemical techniques, neurochemical analysis, and stereological techniques will be used to measure brain lysosomal proteins, alpha-synuclein, and nigrostriatal pathology at multiple ages. The proposed approach is innovative, because it focuses on the interaction between two genetic loci linked to Parkinsonism in an in vivo environment. The proposed research is significant because it is the first step in a line of research expected to lead to the identification of mechanisms contributing to the development of PD and provide a model to test potential therapeutics targeting these mechanisms.